Meaning that something is definitively finished such that whatever it is doing (or being done to it) should or can be stopped, or considered to be done.

From grilling -- not to test whether something is done, b/c that wouldn't make any sense, given what the phrase means. Instead, to stick a fork in the piece of sufficiently-grilled-meat so as to take it off the grill. Connotations of definitive doneness enhanced by sub-conscious notion that the piece of meat is not only completely cooked, it is no longer reasonably conceived of as part of a living animal, and thus can be stabbed with impunity.

Now that Iraq has descended into chaos, you can stick a fork into Bush's re-election chances.